Well this is one of the main reasons we recommend the CB-6000 series to start with, because it comes with five different ring sizes. What we find, which I read is a common experience, is that as you wear them more you find you want a tighter ring to give more security and for a more effective fit.
Once you’ve done that over a month or two then it’ll settle down to a ring size that works for you, and you can then use that to order the ring size for the metal cage!